Horses are similar to many other species with the skin being the largest organ in the body. This is based on overall organ size; just think about the total surface area of a 600 kg horse! The good news is that it can be easily evaluated by visual inspection and palpation. Specific tasks for equine skin include protection from trauma, thermoregulation (temperature control; sweating and heat conservation), sensory perception, secretory function and pigmentation. In addition, it is important to recognize that skin is an important indicator of systemic health since disorders of the skin may actually indicate systemic ill health.

Hives (urticaria)
Of all domestic species horses show evidence of hives most often. The technical veterinary terminology for hives is urticaria. In some cases horses will have a condition of recurrent or repeated bouts of hives. The skin lesion itself is an indication of allergic reaction. This is not a specific disease, yet it is a clinical sign of a systemic problem. In some cases this may be a very serious condition such as after drug treatment, vaccination or insect bite. Other times it is a more mild condition associated with an air-borne allergen. The size of the bumps, how rapidly they develop and the contact they have with one another are all important characteristics of the condition in determining the severity of the problem. Those that occur rapidly, enlarge quickly and touch with one another (coalesce) are the most severe. Figure 1 shows examples of mild (individual) and severe hives (coalescing) lesions in two different horses.
The cause of hives is somewhat complicated. Specific immune cells called mast cells and basophils are the source of the proteins that result in hive formation. The primary protein released from these cells is called histamine. Once the allergen is present in the host, either by contact, ingestion (by mouth) or inhalation (by the respiratory tract) a series of events occurs that results in the release of inflammatory mediating proteins that include histamine. The effects of these proteins are increased leakage from blood vessels, increased recruitment of white blood cells to the area of inflammation and hive or wheal formation.   Immunologic or hypersensitivity reactions to dugs, ingested material, or inhaled pollens or dusts are potential sources for allergen exposure. The most common medication to result in hives is penicillin. Air borne allergens may be certain types of pollens or molds or specific feed types. Alfalfa is a forage source that results in allergy in certain horses, the horse pictured in Figure 1 (lower panel) is suffering from hives as a result of allergy to alfalfa hay.
Definitive diagnosis of the cause of hives can be challenging. Many times symptomatic therapy is required for the initial stages of disease and following anti-inflammatory therapy the lesions will disappear. In cases where the lesions recur, diagnosis may be required. The most effective method of making a diagnosis is to skin test the individual with a series of different allergens. Once the allergen (s) is / are defined they must be eliminated from the horse’s environment. This can be very challenging to accomplish for certain air borne or particulate allergens. In cases where elimination can not be accomplished, symptomatic therapy may be required intermittently. An example of recurrent disease may be associated with certain insects during warm months of the year; lesions will recur as long as insect bites continue to occur. Once the seasons change then the insect population will regress and lesion development will be less common. Horses maintained in warm climate environments may have more problems with clinical disease associated with warm weather insects or pollens.
Treatment with a low dose of allergen over a period of many weeks to months has been used in people and small animals to reduce immune responses to antigen stimulation. This process of reducing immune reactivity is called hyposensitization. Hyposensitization may be attempted in an effort to reduce the reactivity of the horse’s immune system. The response to this treatment may take weeks to many months to see an improvement.
The goal of management of affected horses involves elimination (or control) of exposure to inciting allergens. Symptomatic anti-inflammatory therapy under veterinary guidance is needed when lesions are present. Once the allergens have been identified and controlled development of lesions and clinical sings may be achieved.
Pastern dermatitis
Scratches, sore heel, pastern dermatitis, and grease heel are all names of a similar disorder that affects horses undergoing frequent moist conditions. As a result of race horses requiring frequent baths it is common under training conditions for pastern dermatitis to occur.
Crusting and flaking associated with scratches occurs on the back side of the pastern in one or several limbs (Figure 2). Although it is common to start as a small circular lesion, the lesions often enlarge to become ulcerated lesions on the pastern.    The condition commonly begins on a white limb and then may spread to involve other limbs.
It is important to identify if a topical irritant may be responsible for the initial lesion. If this is the case the irritant material should be thoroughly washed off and the limb(s) dried. Not uncommonly the initial lesion is a minor irritation and secondary complications occur from opportunistic bacterial infections. The secondary complications can make this condition extremely difficult to clear. Local cleansing and keeping the limb dry will often be curative. Thick emollients should not be applied, the most important aspects of managing this condition is keeping the limb(s) clean and dry without adding topical irritants or materials that will retain moisture.
In severe cases antibiotic therapy may be necessary and therefore veterinary consultation is recommended. Severe disease may require additional diagnostic tests to be performed such as skin biopsy for histologic analysis and bacterial / fungal culture. In some instances a definitive cause can not be identified and care with cleaning the limb and keeping it dry will improve the condition.
The goal of managing a horse with scratches is to keep the affected areas clean and dry. Avoid application of topical emollients since these agents will retain moisture in the affected areas. In severe cases veterinary attention may be required to completely clear the infection with antibiotic therapy.
Dermatophilosis
One of the most important skin conditions in horses involves bacterial infection with Dermatophilus congolensis. Characteristics of this condition include matted hair, skin crusting, scab formation and hair loss. Most commonly horses that suffer from this condition are maintained in prolonged moist conditions, occasionally horses that are maintained under dry conditions develop this disorder.
The area where the lesions may be observed are usually over the head, neck, and sides of the belly or chest. More localized disease may occur over the top line (back) region or on the cannon bone of the hind limbs, particularly in race horses.
Signs associated with this condition may be local or generalized and include hair loss and skin crusting, without itching. When the condition is severe horses may show signs such as depression, poor appetite, weight loss or fever. When this condition occurs on the head or legs the areas of white skin are more likely to be affected. When horses have a thick coat the crusting scabs are more easily felt than seen, the crusts are located deep in the coat next to the skin surface.   When the crusts are removed pus material may be observed on the skin or crust. These lesions can also occur on short coated horses during summer months. The cannon bone condition in race horses occurs in warm moist conditions. Small matted hair patches are observed down the front of the cannon bones.
The diagnosis can be made by careful examination, findings that are consistent with those described, and microscopic analysis of the crusts. In suspect cases bacterial culture can be used to confirm the diagnosis.
Treatment of this condition requires careful cleaning of the affected area and removal of scabs. It is important to recognize that this is the result of a skin infection and the skin may be very painful to touch, therefore removal of scab material may be strongly refused by the horse. In severe cases, veterinary aid should be implemented to allow for safe and effective removal of scab material. Gentle washing should include a dilute betadine or chlorhexidine scrub (surgical scrub). In some instances soaking scab material with warm water will facilitate removal and will not meet with severe objection by the affected horse.
Infections that involve white skin may be associated with secondary light sensitivity, so that the horse develops a severe sun burn. Protection from sunlight is recommended in such cases, if the horse must be outdoors, then powerful sun block (SPF 25 or greater) should be applied.
When this condition is present on the lower limbs bandaging should be used with caution. The presence of the bandage material will promote a dark, moist environment, which will prolong healing time. Clean and dry limbs will be most likely to heal. Contact with wet bedding material or surfaces (wet grass) should be avoided until the lesions have healed. Scar formation may occur with longstanding disease; this will possibly lead to skin cracking and reinfection. If lameness occurs at any point veterinary consultation should be sought out immediately.
Dermatophilosis is a skin infection that results in crusting and cracking, which involves the cannon bone in race horses. Mild cases respond well to dry, clean conditions. In more severe cases antibiotic therapy may be required. Caution should be implemented when affected horses are handled because lesions are typically painful to touch, making affected horses potentially dangerous to work with.
Dermatophytosis
Ringworm is a highly contagious fungal skin infection that can affect horses of all ages. Younger horses are more sensitive than order horse, particularly those maintained under stressful conditions. Spores of various fungal species can exist in the environment for extended periods of time; they are remarkably resistant to environmental destruction.   Most cases are observed in the cool winter months, when horses are confined indoors and are groomed extensively.   Outbreaks may also occur at times of warm wet weather, a superior time for fungal sporulation on growth.
Infection is dependent upon live spores and skin abrasion, in some cases following very mild skin trauma. For this reason most lesions occur in the girth or saddle friction areas.
Early signs appear as hairs that stand up off of the skin surface. Hair loss occurs easily at this time and commonly appears as a round area of hair loss. Hair loss occurs in a expanding region with the borders becoming diffuse and ill defined. The girth, neck and shoulder / chest wall are common sites for infection to occur. Generalized infection is not common, but may develop in a young horse that is under severe stress or illness.
The horse will only appear to be itchy on the lesions early in the course of disease (first few days). As the affected area begins to heal hair loss will persist on the outer portion of the affected area, while hair growth will occur centrally in the affected area.
The diagnosis of fungal skin infection is made by microscopic evaluation that reveals the presence of fungal organisms and fungal culture.
Management of horses suffering from fungal skin infection usually involves exposure to sunlight and good nursing care, reduced stress, and maintaining excellent nutrition and proper deworming. Recovery occurs over a period of approximately 2 weeks. Specific treatment is aimed at controlling infection in the individual horse as well as controlling infection in the environment. Clipping affected areas in horses with long hair coats is recommended. Thorough cleaning of clipper blades is required after use on infected horses. Topical administration with an antifungal product containing miconazole is typically effective. All horses in contact with the infected individual and equipment should be monitored for evidence of disease and considered exposed. Exposure to sunlight of other infected horses is necessary as well. Topical cleansing of affected areas with an iodine scrub or 2.5% lime sulfur will also aid in control of the spread of lesions. Oral treatment with antifungal agents should be considered in severe cases that don’t respond to local treatment. Systemic treatment of such cases will require veterinary consultation and evaluation.
One of the most important aspects of managing fungal skin infection is limiting exposure to other horses in the same environment. All hair and contact materials should be considered “infected” material and must be properly disposed. Appropriately diluted washes of antifungal drugs should be used to clean the environment so that continued or reexposure does not occur. Horticultural antifungal fumigants can be used for environmental cleaning, but should not be used for topical use. It is also important to recognize that people can develop lesions from fungal agents and proper protection should be implemented to avoid human infection. Individuals that do not have normal immune function should not work with horses suffering from fungal skin infection.
Summary
In summary there are many skin conditions that can affect horses, particularly race horses that are under stressful conditions. Following good methods of general maintenance and hygiene will reduce the likelihood of lesions developing. If lesions should occur identification of the type of skin infection will aid in the best course of action for management. In cases that don’t respond well to initial attempts at management as well as those that progress despite treatment will require veterinary consultation for additional diagnosis and treatment.

Elizabeth G. Davis, DVM, PhD, DACVIM, Kansas State University  (16 October 2006)

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Dermatologic conditions that affect thoroughbred racehorses

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